The present invention relates to a mercury-free metal-halide lamp comprising no mercury, which is usable for various light sources such as general luminaries, and motor vehicle headlights combined with reflectors and the like.
A lamp used conventionally for motor vehicle headlights has been principally a halogen lamp comprising a tungsten filament. However, a metal-halide lamp, which is a high-pressure discharge lamp of metal halide, has recently been adopted for the purpose of obtaining a high efficiency and improving a recognition of white lines.
In the above-mentioned conventional metal-halide lamp, rare gas, metal halide (solid matter) and, additionally, mercury are enclosed in an arc tube. Rare gas of these enclosures is enclosed principally in order to facilitate a starting of a lamp and to obtain a high light output immediately after the starting, metal halide is enclosed in order to obtain an appropriate light output during a stable operation, and mercury is enclosed in order to obtain a sufficiently high voltage between the electrodes (amp voltage), which is required for the stable operation of the lamp.
A high voltage between the electrodes can be obtained in the lamp in operation particularly by the enclosure of mercury, and thereby the lamp is operated at a low lamp current. As a result, the heat load of the electrodes (Joule loss) is reduced and the lamp can be operated for a long time up to several thousand hours.
A lamp appropriate for motor vehicle headlights, for instance, disclosed in Japanese Unexamined Patent Publication No. 59-111244, is known as a concrete example of conventional metal-halide lamps. The conventional metal-halide lamp according to the Publication will be described below showing in FIG. 16.
In FIG. 16, 101 indicates an arc tube made of quartz, and 102 at both ends of the arc tube 101 indicates a seal portion. 103 indicates a pair of electrodes made of tungsten, 104 indicates a molybdenum foil, and 105 indicates a lead wire made of molybdenum. The electrodes 103 are connected electrically with an end of the molybdenum foil 104 sealed in the seal portion 102, and additionally, the lead wire 105 is connected electrically with the other end of the molybdenum foil 104.
The tips of the electrodes 103 in the arc tube 101 are disposed so that a distance between the tips, namely, a distance between the electrodes is approximately 4.2 (mm). An internal volume of the arc tube 101 is approximately 0.03 (cc). Approximately 0.7 mg(approximately 1.1 mg/cc per unit internal volume of the arc tube) of mercury 106; approximately 0.3 mg in total (approximately 12.0 mg/cc per unit internal volume of the arc tube) of halide 107 composed of sodium iodide, scandium iodide and thorium iodide; and xenon gas with a pressure of 0.7MPa at room temperature, not shown in FIG. 16, are enclosed inside the arc tube 101.
In the above-mentioned metal-halide lamp, the lamp voltage becomes approximately 70 to 80 V Consequently, for instance, in the case of operating at a lamp power of approximately 35W, the lamp current becomes approximately 0.4 to 0.5A.
Thus, a high lamp voltage is obtained by mercury. As a result, the above mentioned conventional metal-halide lamp can be operated at a low current, and thereby this conventional metal-halide lamp has a long life up to approximately two thousand hours.
As described above, the enclosure of mercury brings the increase of the lamp voltage, and thereby a long lamp life up to several thousand hours is provided for us.
However, on the other hand, the above-mentioned conventional metal-halide lamp has a disadvantage of causing high manufacturing costs frequently because of requiring the step of injecting liquid mercury for manufacturing. Moreover, in recent years, metal-halide lamps comprising no mercury have been desired in consideration of the global environment.
However, if mercury is removed from the above-mentioned conventional metal-halide lamp, the lamp voltage drops to approximately 25 V In this case, the lamp current in operation becomes approximately 1.5 A, which is approximately three times as high as a conventional metal-halide lamp wherein mercury is enclosed. Consequently, the heat load of the electrodes (Joule loss) is increased and the evaporation of the electrode becomes active. Therefore, in a mercury-free lamp having a constitution in which mercury is merely removed from a conventional metal-halide lamp, the problem is that the arc tube is blackened in no more than several tens of hours and reaches the end of its life in a very short time. Moreover, since a distance between the electrodes is increased by the evaporation of the electrodes, the operation of the lamp changes while operation time becomes longer, and additionally, an excessive load is caused on the driver circuit.
Since a complete removal of mercury from a fluorescent lamp reduces efficiency greatly on the current level of technology, an effort to decrease mercury is made; however, no mercury in a fluorescent lamp is not yet achieved. In addition, an effort to intend no mercury in a metal-halide lamp is made, and consequently, an electrodeless discharge lamp comprising no mercury is in the process of being available on the market. Meanwhile, an electrode discharge lamp comprising no mercury is still in the stage of study.
In view of the above-mentioned points, the purpose of the present invention is to provide a mercury-free metal-halide lamp wherein a long lamp life can be obtained without enclosing mercury, and causing a rise in the lamp voltage and the blackening of an arc tube 1 by the evaporation of the electrodes while operation time becomes longer.
That is, it has been conventionally considered by experience that a little lamp current is necessary for making the life longer. However, the same life can not always be obtained at the same current depending on the conditions of specifications and operation of the lamp. The inventors of the present invention have found out through a long study that the important factor for the lamp life is not a value of current but current density, and completed the present invention.
The invention according to claim 1 is:
a mercury-free metal-halide lamp comprising a pair of discharge electrodes in an arc tube, wherein at least a rare gas and a metal halide are enclosed in the arc tube, and I/S is 20 (A/mm2) or less on a condition that an area of a cross section at a tip of the above-mentioned discharge electrodes is S (mm2) and a lamp operating current is I (A).
The invention according to claim 2 is:
a mercury-free metal-halide lamp according to claim 1, wherein the above-mentioned I/S is 15 (A/mm2) or less.
The invention according to claim 3 is:
a mercury-free metal-halide lamp according to claim 1, wherein a temperature at a tip of the above-mentioned discharge electrodes is 3200 K or less.
According to these constitutions, a great rise in the lamp voltage and the blackening of an arc tube by a synergistic effect of a rise in vapor pressure of a metal halide and an increase in a distance between the electrodes, resulting from the evaporation of the electrodes while operation time becomes longer, are restrained, and thereby a long lamp life can be obtained.
The invention according to claim 4 is:
a mercury-free metal-halide lamp according to claim 3, wherein a temperature at a tip of the above-mentioned discharge electrodes is 2500 K or more.
According to this constitution, it is easily possible to start a stable discharge.
The invention according to claim 5 is:
a mercury-free metal-halide lamp according to claim 1 or 3, comprising at least one of a scandium halide and a sodium halide in the above-mentioned arc tube.
The invention according to claim 6 is:
a mercury-free metal-halide lamp according to claim 5, comprising at least one of a indium halide and an yttrium halide in the above-mentioned arc tube.
The invention according to claim 7 is:
a mercury-free metal-halide lamp according to claim 1 or 3, comprising at least a trivalent of indium halide in the above-mentioned arc tube.
The invention according to claim 8 is:
a mercury-free metal-halide lamp according to claim 7, further comprising a thallium halide in the above-mentioned arc tube.
The invention according to claim 9 is:
a mercury-free metal-halide lamp according to claim 7, further comprising at least one of a scandium halide and a sodium halide in the above-mentioned arc tube.
The invention according to claim 10 is:
a mercury-free metal-halide lamp according to claim 8, further comprising at least one of a scandium halide and a sodium halide in the above-mentioned arc tube.
The invention according to claim 11 is:
a mercury-free metal-halide lamp according to claim 7, wherein the above-mentioned trivalent of indium halide is at least one of iodide and bromide.
According to these constitutions, since a high lamp voltage can be obtained, it is possible to make current density a low value easily and thereby to prolong the lamp life certainly.
The invention according to claim 12 is:
a mercury-free metal-halide lamp according to claim 1 or 3, comprising an external tube for maintaining the above-mentioned arc tube, wherein a reflection layer of infrared rays is formed on the above-mentioned external tube.
According to this constitution, since the heat retention of the lamp is raised, the vapor pressure of a metal halide rises easily. Consequently, since the lamp voltage can be raised, it is possible to make current density a low value easily and thereby to prolong the lamp life certainly.